1. Field of the Invention
The invention relates to electrostatographic document production machines, and more particularly to automatic adjustment of parameters influencing the output reproduction of such machines.
2. Description of the Prior Art
In electrostatographic document production machines such as printers and copiers, image contrast, density, and color balance (in color machines) can be adjusted by changing certain process control parameters such as primary voltage V.sub.0, exposure E.sub.0, development station electrode bias voltage V.sub.b, the concentration of toner in the developer mixture, and the image transfer potential.
Control of such parameters is often based on measurements of the density of a toner image in a test patch. However, errors caused by densitometer drift due to environmental conditions have plagued the industry. For example, operating characteristic instability (drift) with temperature changes and/or contamination (dusting) by toner particles will greatly affect the accuracy of a densitometer.
U.S. Pat. No. 4,313,671, which issued to H. Kuru on Feb. 2, 1982, addresses the problem of errors caused by environmental conditions. In Kuru, an untoned area in the interframe region of a photoconductive member is compared to a toned reference patch, the difference being an indication of the toning characteristic of the machine. In a first embodiment, different sensors are used for the toned and untoned areas. This has all the disadvantages of two sensors, including initial cost, the need to compensate for inequality in the sensitivity and linearity of the individual sensors, the assumption that both sensors are exposed to same temperature changes and contamination, and the requirement that both sensors react equally to changes in environment.
In a second embodiment, Kuru uses a single sensor to first scan an untoned area and then a toned reference patch in the same interframe region of the photoconductive member. The output of the sensor as it scans the untoned area is compared to a reference value. Any detected error is fed back to adjust the quantity of light illuminating the photoconductive member to correct for sensor variation. While this configuration overcomes the disadvantages of the embodiment having two sensors, it requires a series of iterations of sensing the untoned region, comparing the signal to a reference, adjusting the illumination, resensing the untoned area, and so on until the reading matches the reference value. Since the photoconductive member is moving during this process, the interframe region must be quite large or the system must be satisfied with incompleted adjustments.
U.S. Pat. No. 4,183,657, which issued to L. Ernst et al. on Jan. 15, 1980, provides a toner concentration test by sensing an untoned area and a toned reference patch in the image frame area of a photoconductive member. A reference voltage obtained by illuminating the untoned area with a low intensity, and a sample voltage is obtained by illuminating the toned reference patch with a greater intensity to compensate for the difference in reflectivity of the two regions. By keeping the amount of light reaching the sensor equal, non-linearities in the sensor response do not interfere with accuracy.
However, Ernst et al. are not without problems. The condition of the image receiver will unequally affect the density readings of the untoned area and the toned reference patch. For example, the untoned area density readings may be greatly affected by the amount of wear of the photoconductive member, the existence of scumming, and the presence of photoconductor fatigue. These variables are greatly increased when the patch is in a portion of the image receiver which is repeatedly toned and erased, such as in an image frame area; and non-uniformities in the amount of wear and scumming from one part of the photoconductive member to another may further degrade the system. Ernst et al. also shares with Kuru the disadvantage of requiring several iterations of sensing the untoned area, adjusting the process, and repeating the operation until the readings are within specification.